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Biomagnetic separation has a wide range of applications in life sciences, from cell sorting to protein purification. But we regularly speak to laboratories and companies whose magnetic separation protocols lack necessary information on the key parameter: magnetic force.

Users often focus on factors such as separation time, magnetic bead losses and avoiding irreversible aggregation. However, these factors don't describe the magnetic separation process, and in fact are only products of that process. This makes it impossible to ensure consistent conditions when scaling up or using a different separator.

As a result of this misunderstanding, the most common mistake when scaling up production using classical magnetic separation devices is to use the same magnetic field that was used at smaller volumes. While researchers may assume that a larger, stronger magnet equals a stronger magnetic force, this is not true. A stronger magnet does not necessarily provide a stronger magnetic force, even if it generates a stronger magnetic field near it

In biomagnetic separation processes, the speed of the magnetic beads is the result of the magnetic force acting against the drag force on the beads from the buffer’s viscosity. Magnetic force therefore is a key parameter in these processes. But what is it and how can it be specified?

## What is magnetic force

Magnetic force is defined as the gradient of the product of the magnetic field and the bead ’s magnetic moment. Note that both quantities are vectorial: i.e., they have both intensity and direction. However, for magnetic beads the magnetization of the beads can be considered parallel to the magnetic field. The magnetic force in biomagnetic separation processes therefore depends on:

1. how the magnetic field changes spatially (i.e., the magnetic field gradient)
2. the strength of the magnetization of the magnetic beads.

When you select the conditions for scaling up your magnetic separation process, it’s important to remember that the magnetic force is not related directly to the magnetic field. Instead, the magnetic force is related to the spatial variation of the magnetic field. This means that the intensity of the magnetic field is not the relevant parameter for determining the magnetic force.

If the magnetic field is homogenous, no matter how high the field is, the force is equal to zero and particles will not move but will instead align with the field. A magnet has a force because it generates a field that changes with distance. It is this change that produces the magnetic force.

Diagram showing the effect on magnetic force with a uniform and non-uniform magnetic field.τ is the torque and F the force generated by a magnetic moment m in a magnetic field B. A magnetic force is only generated if the magnetic moment and/or the magnetic field values change on the space, thus its gradient ∇(m·B) is different from zero.

The old saying ‘magnetic beads don’t work at large volumes’ is, simply, wrong. If you want to learn what leading IVD-manufacturers already know, this is the e-book for you!

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